IC card systems are widely known as those for near-field wireless communications. FIG. 20 shows one example of the structures of IC card systems (JP 2010-200061 A). Taking data transmission from a read/write apparatus to a transponder for example, the structure and operation of this IC card system will be explained. A reader/writer 280 (hereinafter referred to simply as “antenna apparatus”) as an apparatus for reading and writing data comprises a first antenna 1a for near-field wireless communications, which radiates electromagnetic waves to form a magnetic field around the antenna apparatus 280. When an IC card 285 as the transponder is made close to the antenna apparatus 280, a second antenna 1b for near-field wireless communications in the IC card 285 is magnetically coupled to the first antenna 1a, so that power is supplied to an integrated circuit 68 by electromagnetic induction, and data transmission is conducted according to protocol set in advance between the antenna apparatus 280 and the IC card 285 (for example, ISO 14443, 15693, 18092, etc.).
The antenna apparatus 280 comprises a semiconductor 70, a first filter (noise filter) 71, a matching circuit 72, and a second filter 73. The semiconductor 70 comprises a transmission circuit, a receiving circuit, a modulation circuit, a demodulation circuit, a controller, etc. An antenna resonance circuit 66 comprises the first antenna 1a for near-field wireless communications, a resonance capacitor 65, and a resistor (not shown). The resonance frequency of the antenna resonance circuit 66 is set to be an intrinsic frequency (for example, 13.56 MHz) used for communications, in which a real part of impedance of the antenna resonance circuit 66 is substantially in a short-circuited state. The antenna resonance circuit 66 is connected to the semiconductor 70 via the impedance-matching circuit 72.
An output terminal Tx connected to the modulation circuit in the transmission circuit in the semiconductor 70 is connected to the impedance-matching circuit 72 via the first filter 71 for EMC. An input terminal Rx connected to the demodulation circuit in the receiving circuit in the semiconductor 70 is connected to a connection point of the first filter 71 and the impedance-matching circuit 72 via the second filter 73 comprising series-connected resistor and capacitor.
The transmission circuit and the receiving circuit in the semiconductor 70 are controlled to be an operation state or a non-operation state by the controller. Signals having a frequency (for example, 13.56 MHz) corresponding to a tuning frequency are supplied from an oscillator to the transmission circuit, the signals being modulated according to a predetermined protocol and supplied to the antenna resonance circuit 66. The first antenna 1a for near-field wireless communications in the antenna resonance circuit 66 is magnetically coupled to the second antenna 1b for near-field wireless communications in the IC card 285 at a predetermined coupling constant, thereby transmitting signals (carrier signals) to the IC card 285. Signals (carrier signals) from the IC card 285 are received by the receiving circuit in the semiconductor 70, after suppressed by a resistor in the second filter 73.
The antenna for near-field wireless communications (hereinafter referred to simply as “antenna”) used in such system comprises, as generally shown in FIG. 21, a coil 10 spirally wound on a surface of a substrate 410. This antenna 1 is called a flat coil, suitable for height reduction. When high-frequency current is supplied to the coil 10, a substantially uniform magnetic flux is generated on the coil side and its opposite side with the substrate 410 as a boundary. However, because only a magnetic flux on the coil side contributes to communications, and because the magnetic flux does not reach far, communication is achieved in a short distance. Hereinafter, a side on which the magnetic flux is used for communications is called “transmission surface side,” and a side on which it is not used for communications is called “non-transmission surface side.”
In a wireless communications apparatus, a metal shield formed by a metal sheet or case, etc. is usually disposed near the antenna 1. In this case, parasitic capacitance is formed between the coil 10 and the metal shield, so that eddy current is generated in the metal shield, reducing the inductance of the coil 10, and changing the resonance frequency of the antenna 1. Further, eddy current loss is generated, making it necessary to increase electric supply to the coil 10 for compensation, resulting in increased battery consumption. In addition, the magnetic flux not contributing to communications acts as noise to other parts, likely causing troubles.
Against such problems, the attachment of a high-permeability magnetic member to a non-transmission side of an antenna is proposed (JP 2004-166175 A). FIGS. 22(a) and 22(b) show a reader/writer antenna 1 having such structure. The antenna 1 comprises a magnetic plate member 30 formed on a metal shield 26, and a coil 10 attached to an upper surface of the magnetic plate member 30. Because a magnetic flux 250 generated by the coil 10 passes mainly through the magnetic member 30, the magnetic flux does not spread on the side of the magnetic member 30 (non-transmission surface side), and reaches far on the opposite side of the magnetic member 30 (transmission surface side), thereby having directivity. The magnetic member 30 between the metal shield 26 and the coil 10 prevents the formation of parasitic capacitance, and reduces eddy current generated in the metal shield 26.
The transmission of power and data by electromagnetic induction has long been known. For example, in noncontact-charging antennas, coils formed by enameled wires are fixed to magnetic member surfaces. To handle larger power than in small-power wireless communications (for example, to supply current of about 1 A to the coils), enameled wires of about 1 mm in diameters are generally used, with coil ends not fixed for flexibility.
Antennas for small-power wireless communications constituted according to the structure of the noncontact-charging antennas have been found to suffer the following problems. Because the antennas for small-power wireless communications handle power of at most about 15 mA, conductor wires having as small diameters as 100 μm or less can be used, and the formation of coils is easy. However, when the coil ends are free, thin lead wires are easily deformed by a small external force, restricting connection methods to other circuits. Also, when the antennas are bent or disposed on curved surfaces, tension is applied to the lead wires, likely resulting in the breakage of conductor wires of the coils, and the unwinding of coils.
Though the coils can have thick conductor wires for increased strength, the coils become thick, particularly in portions overlapping the lead wires at their ends. The antennas become thicker as the conductor wires have larger diameters. When used for small wireless communications apparatuses such as mobile phones, thin, small antennas are preferable, so that substrates should have slits for receiving lead wires to prevent the thickness increase of antennas.
In applications of limited thickness such as IC card systems, coils should be as thin as possible to provide easily handleable, thin antennas resistant to breakage, etc. Thus proposed are the formation of a coil called “printed coil” on a flexible substrate by etching a metal foil or a vapor-deposited metal film in place of using a conductor wire such as an enameled wire (JP 2004-166175 A), and the production of an antenna by printing a conductive paste in a coil shape, and transferring the resultant coil-shaped conductor pattern onto an adhesive film. However, the printed coil needs a patterning step, an etching step, etc., and the transfer-printed coil needs a printing step, a transferring step, etc., resulting in more expensive coils than those constituted by conductor wires.
In addition, because the printed coil has a thickness of about 30 μm, it should be wide to have smaller electric resistance to avoid the deterioration of antenna characteristics such as a Q value, etc. Accordingly, with the same number of winding, the printed coil occupies a larger area than that of the conductor wire coil, preventing the miniaturization of antennas. The reduction of the number of winding of a coil for being received in a predetermined size results in inductance decrease and a smaller communication distance. Though the conductor pattern can be made thicker, coils become more expensive accordingly.